This application claims the priority of Korean Patent Application No. 2002-61214, filed Oct. 8, 2002, which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to optical clock recovery, and more particularly, to a method and apparatus of clock recovery using optical filters with fixed wavelengths.
2. Description of the Related Art
In order to increase the data transmission speed in an optical transmission system, a technique associated with an optical transmission unit for converting transmission data into an optical signal and transmitting the optical signal must first be improved, and a technique associated with an optical receiving unit for receiving a transmitted signal and restoring it to the original signal must also be improved in advance. Additionally, the signal processing speed in the optical receiving unit must be fast enough to prevent bottleneck. To demodulate a transmitted data signal at a high speed and with accuracy, optical clock recovery has been studied.
Examples of an optical clock recovery method being studied at present are a method using a self-pulsating phenomenon, which occurs in a laser diode, and a method using an optical loop mirror. However, these methods have disadvantages in that it is difficult to precisely manufacture a clock recovery device such as a phase locked loop (PLL) used for accurate clock recovery and that an optical transmission system is unstable because of the optical fiber that is very sensitive to changes in external environments, for example, a temperature change in the outside. To solve these problems, various methods have been disclosed.
Among the above-disclosed methods, there is a method of recovering a clock signal using particular spectral components that exist on the optical spectrum of a received optical signal. It can be seen from the optical spectrum of a received optical signal that the optical spectrum contains a few spectral lines whose magnitudes are relatively larger than those of the other spectral lines. A clock signal is recovered by extracting the largest spectral line at the central frequency and one of side spectral lines adjacent to the largest spectral line from the above-mentioned spectrum.
To be more specific, the magnitudes of two extracted spectral lines of the optical signal should be made equal by passing them through filters and an attenuator. Then, the resulting optical signal is received at a detector and is then subjected to beating in order to derive a signal corresponding to the frequency difference between the two spectral lines. The signal derived from the received optical signals through the beating process has the same frequency as the clock of the received optical signal. The received optical signal may be either a non return to zero (NRZ) type signal or a return to zero (RZ) type signal.
In the prior art, a tunable filter is used to extract the desired spectral lines and to make the magnitudes of two extracted spectral lines equal to each other. The tunable filter adjusts the magnitudes of two spectral lines, which can be different for each system due to the characteristics of an optical transmitter used in the optical transmission system, so that the magnitudes of the two spectral lines are made equal to each other.
However, the use of only one tunable filter have problems that the signal to noise ratio (SNR) of the system is limited by the small noise components that exist between the two extracted spectral lines in the optical spectrum and that a clock recovery device is expensive.
Furthermore, a tunable filter can make the two spectral lines equal in magnitude in certain range of the magnitude difference between the two extracted spectral lines. The noise components that exist between the two spectral lines are also extracted when the two spectral lines are extracted, thus the extracted noise components affect as noise that are also subjected to beating. Therefore, the performance of a clock recovery system can be degraded.